An organic EL element includes an organic light-emitting layer between a plurality of layers of electrodes to electrically obtain light emission. In addition to the use thereof as a display device as a replacement for liquid crystal cells, the organic EL element has also been studied for applications as a surface light source device such as a flat-type lighting apparatus and a liquid crystal display backlight, utilizing characteristics such as high emission efficiency, low voltage driving, light weight, and low cost.
When using an organic EL element as a light source of a surface light source device, light has to be extracted from the surface light source device in a useful manner with high efficiency. To this end, in prior art surface light source devices, it is known to provide a variety of concavo-convex structures on the light output surface side of the organic EL element. A lot of studies have been made regarding such concavo-convex structures for surface light source devices, particularly devices of single-side emission type where light is extracted from one side (for example, FIGS. 4 and 6 of Patent Literature 1, etc.).